Support "Key Chords" in keybindings

Currently, we only support pressing a single key+modifiers to activate a keybinding action.

However, many text editors support "key chords" where the chord is a combination of multiple keys pressed in sequence. For example, in Visual Studio, the default keybinding for "Comment Code" is the chord [ctrl+c, ctrl+k].

We've already prepared for serializing these chords in the array of keybindings, but we don't support more than one key at a time. We'd have somehow check if a key is the start of a chord, and only dispatch the action when all keys for the chord have been pressed.

Lots of questions:

• What happens if an action is bound to [ctrl+c, ctrl+k] and another is bound to [ctrl+c]?
• If the first key of a chord is pressed, but the second key isn't bound to an action, should we write both keys to the input?
• If the first key of a chord is pressed, but then nothing is pressed for a long time, should we time out?
• What's the best way of finding the right keybinding for a chord?
  • Iterating over all of them when a key is pressed, until the chord is dispatched seems awful (but trivial to do).
  • Maybe we could build a tree of keychords, or a list of trees?
  • My CS542 senses are tingling and suggesting that this might be the case to use a state machine.
    • Is a state machine really different than a tree in this case?